Since the publication of the paper: FFC, “First Report and Order: Revision of part 15 of the commission's rules regarding ultra-wideband transmissions systems,” FCC,ET Docket 98-153, April 2002, by the United States Federal Communications Commission (FCC), Ultra-Wideband (UWB) communication techniques have received increasing attention for short-range, high-data rate wireless communication applications. Impulse Radio (IR) implementation of such UWB systems has become a very interesting candidate due to its low complexity, low power consumption, low cost, high data rate and the ability of coexistence with other radio systems.
In its usual realization, an IR-UWB transmitter consists on a pulse generator that is triggered regularly by a timing circuitry, that is the IR-UWB is based on the transmission and reception of very short pulses at a certain periodicity. The output of the pulse generator is connected directly to the antenna. No Power Amplifier is needed since the UWB transmitted power is very low. Data is transmitted by modifying some parameter of the pulse (for example, its sign in BPSK modulation or its position in PPM modulation). The transmitted output waveform has a very small duty cycle since the sub-nanosecond pulses are sent every frame, which for usual data rates has a duration of several nanoseconds.
Time Hoping (TH) technique is commonly used to allow multiple users access and to avoid peaks in the spectrum of the UWB signal. A pseudorandom code locates each successive pulse in a different position along its frame. The IR-UWB receiver can be implemented in several ways, but the present invention is focused on a very low-power implementation for short distances (less than 1 meter for Body Area Network, BAN, applications). A possible architecture for such IR-UWB receiver is shown in FIG. 2.
This architecture corresponds to a coherent receiver that decides the output data symbol after integrating the result of the multiplication of the received signal with a locally generated template waveform. A Low Noise Amplifier (LNA) is needed at the input, in order to adapt the antenna impedance to the input and to minimize the noise of the receiver chain by providing enough gain. The LNA has to add low noise by itself to the received signal.
As the received waveform has a very small duty cycle, blocks such as the LNA, the multiplier or the integrator do not need to operate continuously, just when the pulse is received. By applying a clever ON/OFF switching on these blocks an important saving of power consumption can be obtained. Additionally, the receiver chain is built fully differential in order to minimize the influence of the substrate noise coupled from the digital section of the receiver (to be integrated in the same die) as well as to reduce the undesired effects due to the coupling of the ON/OFF switching signal itself.
As in other systems that work in burst mode, IR-UWB receiver front-ends can ideally work only when a pulse is expected to arrive. Switching ON and OFF the LNA is a known technique that allows an important power saving. For example, assuming a received pulse of duration 1 ns, and a data rate of 100 Mps, an ideal power reduction of 90% can be achieved if the front-end blocks are turned ON only during the pulse time. In practice the power saving will not be so high, as the LNA must be switched ON some time in advance to the reception of the pulse in order to be completely stabilized when the pulse arrives.
A LNA with power switch is described in the technical paper entitled “Transceivers circuits for pulse-based ultra-wideband” in the IEEE Journal 0-7803-8251-X/2004 IEEE. FIG. 1 represents an analog circuit of a LNA with power switch described on said paper, in which the power consumption is reduced by switching the bias current, ‘which is the advantage of the discontinuous feature of the pulse based UWB.
The LNA of FIG. 1 uses a technique of switching OFF and ON one of the active devices of each branch by acting over the gate of said devices, to interrupt the flow of current between VDD and GND. This is done typically using NMOS cascode/cascade transistors because their gate can be DC powered to VDD in the ON state. Shorting it to ground in the OFF state effectively powers down the amplifier. More in detail, in the LNA of FIG. 1 the circuit is powered down by means of transistors (Tr1), (Tr2) connecting to ground the gates of the transistors which receive a RF input signal (RFin). In that switching scheme, the transistors (Tr1), (Tr2) used to perform the on/off switching, interfere with the input signal path and the input matching network and consequently negatively affect the LNA gain and linearity. Transistor (Tr3) is used to disconnect the whole amplifier from the power supply.
On the other hand, a Capacitive Cross-Coupling (CCC) technique is known to improve the performance of a differential amplifying stage. More details of this CCC technique can be found on the papers: “Wei Zhou, Sherif Embabi, José Pineda de Gyvez and Edgar Sanchez Sinencio, “Using Capacitive Cross-Coupling Technique in RF Low Noise Amplifiers and Down-Conversion Mixer Design”, in the Proceedings of the European Solid-State Circuits Conference(ESSCIRC), Sep. 19-21, 2000 Page(s):77-80”, and “David J. Allstot, Xiaoyong Li, and Sudip Shekhar, “Design Considerations for CMOS Low-Noise Amplifiers”, in IEEE Radio Frequency Integrated Circuits Symposium (RFIC), 2004, pp. 97-100”.